Efficiency of rotating aircraft turbine engines (as opposed to reciprocating) depends partially on the efficient air compressor section which, in turn, depends on the clearance control between blade tips and the adjacent surfaces. The analogy would be the fit up and clearance of a piston ring and cylinder wall for an efficient compression stroke.
Because of the close assembly tolerances and the thermal expansion of the parts, a close clearance is maintained by having an abradable material opposite the tips of the blades. The blades cut their own groove or path, thereby minimizing leakage of air around the tips of the blades. Each engine has rows of moving blades mounted on rotating disks with rows of stationary blades in between each rotation section to redirect the flow into the next rotating stage. The stationary blade tips cut their groove into the rotating seal attached to the rotating compressor disk.
The performance of the abradable seal material is such that while it must be abradable and not abrade the tips of the blades, it cannot stick to the blades. It must also be able to stand the elevated temperature due to compression as well as the erosion of abrasive particles injected into the intake air. The dynamic seal material must also have excellent bond strength to withstand the centrifugal forces induced at 30 to 40,000 rpm. Moreover, when the unit is stopped and parts are in contact, a lower coefficient of friction would facilitate starting by reducing drag forces at the interfaces of the seal material and blade tips.
In titanium bladed auxiliary power units (A.P.U.) a problem exists that when the unit remains stationary, particularly in humid and/or salty atmospheres, the titanium blades tend to seize against the abradable seal. This is because the abradable seal material is about 85% aluminum, or aluminum bronze, which are very much apart on the anodic chart with titanium and hence there is accelerated galvanic-type erosion of the aluminum alloy causing a seizing at the seal interface. The solution appeared to be in a non-metallic abradable seal material.
The present commercial system that has been widely accepted by the aerospace industry is a system which calls for thermal spraying a mixture of aluminum powder and resin (85% aluminum and 15% resin). The process includes grit blasting the surface, applying a bond coat by thermal spray for better adhesion, then thermal (plasma) spraying the aluminum or aluminum bronze powder, machining this deposit to dimensional requirements, then a seal coat of a resin is applied to impregnate the sprayed deposit. Because the spray process must spray the aluminum powder and resin separately into the plasma, there at at least 15 variables and Taguchi Statistical Process Control methods are constantly applied every time some of these variables are changed.
In other similar systems, the material is also an aluminum or aluminum bronze spray powder with a polyimide powder rather than a polyester powder. None of the versions address the galvanic problem.
A Teflon.TM. (a synthetic resin polymer, solid lubricant) coating is placed over a high temperature resin microsphere mix after the substrate has been machined to allow a coating of 5 mils to be added to the surface. This type of abradable seal solved many of the above problems. However, if the Teflon.TM. particles on the surface were fully nucleated or melted together rather than just sintered, the coating in the thicker areas would tend to peel rather than abrade. Moreover, due to the dimensional tolerances, and difficulty and economics of applying a thick (.003" or over) coating of Teflon.TM., it was more realistic to assume that the wear of the blade tips would penetrate into the substrate.
In a preferred embodiment of the invention the Teflon.TM. coating is eliminated and solid lubricant Teflon.TM. is incorporated into the microsphere and high temperature resin mix.
From wear tests and metallographic examinations of the surfaces, it appears that some of the Teflon.TM. or solid lubricant particles embedded in the resin break loose and are trapped in the cavities, nooks and crannies of the microspheres on the surface, thus producing a lower friction force. The friability and the frangibility of this system enables the wear to take place in the abradable seal material without wear or adherence to the titanium blade tips. This system has about 1/3 the variables that are in the present system and does not require the use of an expensive plasma or high velocity thermal spray system and associated equipment.
In a further preferred embodiment, ceramic fibers are incorporated into the resin, microsphere Teflon.TM. mix.
The basic objectives of the invention are to provide an improved abradable seal for rotating turbine engines and a method of producing same.